Except if my opponent doesn't move, Highland Unicorn won't get Charge +2. And the Regenerate 1 is only helpful if I have more creatures sharing its zone. Knight of Westlock's attack is 5 dice even without having to move, and While the Unicorn has regenerate 2, knight of westlock has 1 more armor and 1 more health than Unicorn. I suppose it could be a useful guard for my mage to heal a bit of damage, but I don't think that's worth replacing the Knight with it. Now, if I was playing in a lightning heavy metagame, then I probably would choose unicorn over knight.